**Core Concept**
The Employees' State Insurance (ESI) scheme is a social security and health insurance program for Indian workers, providing financial assistance and medical benefits to employees in case of sickness, maternity, disability, or death. The ESI scheme has specific rules regarding the duration and eligibility for sickness benefits.

**Why the Correct Answer is Right**
Sickness benefit under the ESI scheme is available for a maximum period of 91 days in a year, subject to certain conditions. This benefit is calculated at a specific rate, usually 70% of the average daily wages, and is payable for a maximum of 91 days in a year. The duration of sickness benefit can be extended in cases of contagious diseases or where the insured person is undergoing medical treatment in a hospital.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not specify the correct duration of sickness benefit under the ESI scheme.
* **Option B:** This option is incorrect as it does not match the actual duration of sickness benefit under the ESI scheme.
* **Option C:** This option is incorrect as it is not the correct duration of sickness benefit under the ESI scheme.

**Clinical Pearl / High-Yield Fact**
It is essential for medical professionals and administrators to be aware of the specific rules and regulations of the ESI scheme, including the duration of sickness benefit, to ensure timely and accurate payment of benefits to insured employees.

**Correct Answer: C. 91 days**